Vendors show voice call hand-off between LTE, 3G networks

Qualcomm this week revealed a key step in enabling voice calls over LTE handsets. Working with Ericsson, the chipmaker says it recently completed the first voice call handover between LTE and 3G networks, and will showcase the achievement later this month at Mobile World Congress in Barcelona.

The hand-off was made on Dec. 23, using a handset with a Qualcomm chipset to pass a voice call from an Ericsson LTE network to a WCDMA 3G network, via a standard called Single Radio Voice Call Continuity, or SRVCC. The achievement shows a viable blending of emerging LTE networks with existing 3G networks for voice.

NETWORKING'S HOTTEST ARGUMENTS: LTE vs. WiMAX 

Voice-over LTE, or VoLTE, is still in its infancy: the advanced cellular network today is mainly for data "calls." LTE smartphones today make use of two baseband chips, one for LTE data calls, and a separate one for voice calls that are delivered via 3G or 2G connections. VoLTE is still missing in action. Even Verizon, the most aggressive of U.S. carriers in deploying LTE, doesn't yet implement it. It began testing VoLTE in 2011, but still won't say when it will be ready. 

The Qualcomm-Ericsson demonstration used a handset with Qualcomm's Snapdragon S4 system-on-chip, with the vendor's newest mobile station modem chipset, MSM8960, which is smaller and higher performing than the previous generation, more power efficient, and adds LTE to its mix of 3G standards for the first time.

Introducing SRVCC is the latest step in blending the burgeoning LTE networks with the existing stock of 3G networks. In 2011, vendors launched what's called circuit-switched fallback technology, or DSFB, which lets a single handset radio switch between an LTE data connection and 3G. SRVCC does the same thing for voice calls.

Together, the capabilities also eliminate the need to use separate LTE and 3G radios and modems, according to a Qualcomm statement. That, in turn, means handsets that use fewer components and less power, and that could be thinner and more compact.

Beyond the voice hand-off feature, the demonstration also showcases the latest Qualcomm baseband components, which are due for volume release in the second quarter. The MSM8960 incorporates a second-generation modem based on the 3GPP Release 9 specification, with several improvements over Release 8, which was the basis of the current crop of LTE smartphones (see the very thorough discussion of this at AnandTech, by Brian Klug and Anand Lal Shimpi): much better performance, more efficient power use, and integrated LTE.

The same modem technology is part of Qualcomm's separate mobile data modem product line. For LTE data connections, this is the new MDM9x15 chip family, also due out in the second quarter. It, too, uses the smaller 28-nanometer silicon die process found in the MSM8960. As Lal Shimpi noted in a separate post, "Apple (and all other smartphone makers) could replace the MDM6600 [currently used in the iPhone] with the MDM9615 and have a "single chip" LTE solution for smartphones. I put single chip in quotes because there are obviously other components necessary such as a PMIC and in the case of the MDM9615, an external transceiver. But next year (Q2 to be exact) should be when we can finally get LTE into something iPhone-sized."

John Cox covers wireless networking and mobile computing for Network World.

Twitter: http://twitter.com/johnwcoxnww

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